1. Field of the Invention
The present invention refers to a power supply apparatus for use in a mobile instrument such as a mobile phone driven by a secondary (rechargeable) battery, and in particular, to a power supply apparatus employing a backup battery capable of supplying power in place of a main battery when the main battery voltage decreases to a prescribed level.
2. Discussion of the Background
In a mobile instrument such as a mobile phone employing a secondary battery as a power supply, it is now possible, by separately providing a backup battery, to maintain the mobile instrument in a prescribed operable condition even when a voltage of the secondary battery (as a main battery) is lower than a prescribed level. In order to prolong a life of such a backup battery, a power supply circuit is generally utilized to prevent consumption of power and reducing a backup time period capacity by prohibiting needless backup battery usage as discussed in Japanese Patent Application Laid Open No. 10-145527 and as illustrated in FIG. 7.
Specifically, a diode 105 is connected between first and second power supply terminals 102 and 104. The first power supply terminal 102 connects to a main battery 101 formed from a secondary battery that supplies the mobile instrument with power. The second power supply terminal 104 connects to a backup battery 103 that supplies power and maintains the mobile instrument in a prescribed operative condition when the main battery 101 voltage Vm decreases to a prescribed level. The diode 105 prolongs a life of the backup battery 103 by preventing current from flowing from the backup battery 103 to the main battery 101.
A control switch 106 is connected in parallel across the diode 105. A voltage monitor circuit 107 monitors the voltage of the main battery 101 and controls the control switch 106 to operate in accordance with the voltage Vm. For example, when the voltage Vm of the main battery 101 ranges from 3.2V to 4.2V as a normal condition, and that of the backup battery 103 is 3.0V, the voltage monitoring circuit 107 turns OFF the control switch 106. When the control switch 106 is turned OFF, the diode 105 is connected between the first and second power supply terminals 102 and 104, and thereby current is prevented from flowing from the backup battery 103 to the main battery 101. As a result, the life of the backup battery 103 is prolonged.
Further, in order to prolong a backup time period (capacity) for a mobile instrument, a voltage of a backup battery recently has been increased. For example, the backup battery 103 having a voltage of from 3.0V to 3.3V is utilized to prolong the backup time period for the mobile instrument.
In such circumstances, however, when the control switch 106 is turned ON and if the voltage Vm of the main battery 101 becomes lower than that of the backup battery 103, some current flows from the backup battery 103 to the main battery 101, whereby the effective life of the backup battery 103 is shortened.